Emulsion explosive compositions are well-known in the art. As used herein the term "emulsion" refers to a water-in-oil emulsion comprising an inorganic oxidizer salt solution as a discontinuous phase and an organic liquid fuel as a continuous phase. See for example, U.S. Pat. Nos. 4,474,628; 4,820,361 and 4,931,110.
Most emulsion explosive compositions have their natural densities purposely reduced to thereby increase their sensitivity to a desired or required level for detonation. Sensitivity often is increased to a level to allow the composition to be detonable by the initiating shock produced from a commercial blasting cap or detonator. Such compositions are commonly termed "cap sensitive." Other compositions are designed to be detonated by a booster or primer rather than detonators, and thus their sensitivities are designed to be at a lower level. Compositions having this lesser degree of sensitivity are deemed non-cap sensitive and are commonly called blasting agents. The more sensitive compositions are generally made or used in smaller charge diameters in either packaged or bulk form.
The commonly used means of density reduction is the addition of density reducing agents to an already formed emulsion. Such density reducing agents include hollow glass or organic microspheres, porous ammonium nitrate (AN) prills, perlite and chemical gassing agents, such as sodium nitrite that decompose chemically in the composition to produce a dispersion of gas bubbles throughout the composition. Air bubbles also can be entrained during mixing or stirring of the composition.
The use of solid, hollow microspheres often is the preferred form of density reducing agent, since they are less compressible under pressure (such as in deep bore holes), they can be incorporated into the composition in desired quantities by relatively simple mechanical means as is known in the art, and they remain stable and distinct rather than being prone to migrate and coalesce as air or gas bubbles do, especially during movement or handling of the emulsion. Their use is not without problems, however, since they are relatively expensive and they present some pre-incorporation handling problems. Because of their low bulk density (typically 0.3 g/cc or lower), they are relatively expensive to transport. Because of their small particle size (average particle diameter of 40-70 microns) and lower density, they can present difficult handling problems. For example, the microspheres easily can become an airborne dust problem if they are stirred or otherwise perturbed in an open environment, potentially creating health and safety hazards. These transportation and handling problems significantly are minimized by the methods of the present invention which allow for the in situ expansion of organic microspheres during the emulsion formation process.
The microspheres used in the present invention are unexpanded organic microspheres, which can be supplied in wet or dry form. organic microspheres have an advantage over glass microspheres in that organic microspheres also act as a fuel that is consumed in the detonation and thus contribute to the energy of the explosive. They typically are heat expanded, however, prior to their incorporation into an explosive composition, and thus the handling problems described above must be addressed. Further, this pre- expansion and drying (if done) requires specialized, costly equipment and processing. Also, the particle size and volume of unexpanded microspheres are significantly less than for expanded microspheres (of any type) and thus the unexpanded microspheres can be transported more efficiently and cost effectively.
The present invention comprises the addition of unexpanded microspheres to a heated component of the emulsion explosive composition during the emulsion formation process. This allows for the in situ expansion of the microspheres in the emulsion explosive composition and thus eliminates the handling and transporting problems described above and the need for costly expanding, drying and other processing equipment. Additionally, it has been found that the detonation velocity is increased by this method as compared to the use of conventionally expanded microspheres.
The in situ expansion of organic microspheres in certain types of explosive compositions has been suggested in the prior art. U.S. Pat. No. 3,773,573 suggests that microspheres may be incorporated into a pre-formed water gel explosive in an unexpanded form and expanded in situ. U.K. patent no. 2,010,239 B discloses the heating and expanding of organic microspheres in an aqueous oxidizer salt solution or eutectic melt prior to combining with the fuel and other components to form a water gel explosive. In a somewhat different approach, U.S. Pat. No. 5,540,793 discloses the addition of microspheres to porous prilled AN during the prilling process. The microspheres disclosed are of various kinds and the organic microspheres can be expanded during the prilling process.
The methods of the present inventions differ from these prior art disclosures in that the in situ expansion occurs during the formation of a water-in-oil emulsion explosive composition. It surprisingly has been found that such in situ expansion occurs with a high efficiency and at significantly lower temperatures when compared to organic microspheres that are pre-expanded using conventional techniques prior to incorporation into an explosive. In addition, a method of the present invention allows for only a portion or component of the emulsion explosive composition to be heated to the expansion temperature rather than the whole composition, and thus this method is more energy efficient and enhances emulsion stability since the final composition is at, or can be cooled more easily to, a lower temperature which is conducive to stability.
Thus one object of the present invention is to provide a method for the in situ expansion of organic microspheres in an emulsion explosive composition. Another object is to provide a method for in situ expansion of organic microspheres in an emulsion explosive composition, which method is energy efficient and enhances final emulsion stability. Another object is to provide a method for improving the efficiency of expansion of organic microspheres at lower temperatures during the formation of an emulsion explosive composition.